Liquid process and continuous filtering device using high power density ultrasounds

ABSTRACT

The present invention relates to a continuous filtration device in a liquid path, characterized in that it comprises, in combination:
         a modular ultrasonic treatment unit ( 12 ) which is in the form of a tubular metal body ( 11 ) with a cylindrical internal surface ( 13 ) and of circular cross section, open at both its feed end ( 10 ) and its discharge end ( 15 ), the external surface of the said tubular metal body having, in the vicinity of the nodal zone, a collar ( 17 ) which is coaxial with the said tube and which projects radially, this collar being equipped at its periphery with an ultrasonic converter ( 19 ) which projects radially and whose frequency is equal to the vibration frequency of the said collar ( 17 ) and to the longitudinal vibration frequency of the said tubular metal body ( 11 ), and   a filtering cylindrical element ( 14 ) having a minimum filtration area of about 50 cm 2  and preferably about 80 cm 2  with a mesh opening less than about 20 μm, which is arranged inside the tubular metal body ( 11 ) and coaxially with the latter between its feed end ( 10 ) and discharge end ( 15 ),
 
the nominal ultrasonic power density dissipated inside the tubular metal body being greater than about 2 watts per cm 2  of filtering area.

The present patent application is a non-provisional application ofInternational Application No. PCT/FR02/00889, filed Mar. 13, 2002.

The present invention relates to a continuous filtration device in aliquid path using high-power-density ultrasound.

More particularly, the present invention relates to a continuousfiltration device in a liquid path using a tubular modular ultrasonictreatment unit acting both as an external jacket for a cylindricalfiltering element and as a source of high-power-density ultrasound.

Such a modular treatment unit is described in a detailed manner inFrench Patent FR 2 671 737 and in the corresponding European Patent EP 0567 579.

In the field of filtration in a liquid path, a certain number of devicesare already known comprising, inside a filtration vessel through which aliquid flows, filtering elements through which the liquid passes andwhich retain the particles to be filtered.

Also known are devices for cleaning and regenerating filtering elementsby periodic application of a counter-current of liquid, possiblysimultaneously with applying ultrasonic vibration within the saidfiltration vessel. This vibration may be created by an ultrasonicvibration source attached to the said vessel, or else immersed insidethe said vessel, close to the filtering elements.

The cleaning effect obtained by virtue of the ultrasonic vibration isdue to the physical phenomenon of cavitation in liquids, well known to aperson skilled in the art.

Filtration devices are also known in which ultrasonic vibration iscontinuously applied during the filtration process, assisting thefiltration.

For example, it is possible to force the passage of particles with asize less than the opening of the mesh or of the pores of the saidfiltering elements with the aid of a straight cylindrical sonotrode suchas those commonly used in the laboratory, provided that the end of thesonotrode is sufficiently close to the said elements so that theamplitude of the vibration obtained in the liquid in contact with thesaid filtering element is large enough.

Such a device remains limited to laboratory applications, or to very lowflow rates, and therefore does not lend itself to in-line use in amanufacturing process, because of the geometry of the sonotrode whoseuseful vibrating surface is necessarily too small.

Devices are also known where ultrasonic vibration is continuouslyapplied to a chamber in which the filtering elements are situated, butwhich is not acoustically tuned to the elements for creating and/orpropagating the said vibrations.

Such devices are limited in the useful acoustic power which can betransmitted to the filtering elements and do not allow a uniformdistribution of this power to be obtained over the filtering elements.

There are other sources of ultrasonic vibrations which can be immersedin liquids, such as dip-tubes. The latter have a large useful vibratingsurface. However, they generate a cavitation power density which islimited and often insufficient (less than 1 W/cm² of filtering surface)to substantially improve a filtration process.

Furthermore, since in this case the filtering element can only be placedoutside the ultrasound tubes, the plant has the drawback of necessarilyhaving to be bulky and of necessitating the use of large-diameterfilters and an external tubular jacket.

At present, there is therefore no filtering device in a liquid pathsatisfying the following various constraints:

-   -   continuously using ultrasonic vibration to improve the        filtration process;    -   obtaining a uniform distribution of the ultrasonic cavitation        power over the entire filtering surface;    -   obtaining, in contact with the surface, a mean ultrasonic power        density much greater than 2 watts per cm² of filtering surface;    -   being compact;    -   being suitable for industrial use.

The aim of the present invention is specifically to satisfy all theabove criteria.

According to the present invention, the filtration device comprises amodular ultrasonic treatment unit which is in the form of:

-   -   a tubular metal body with a cylindrical internal surface and of        circular cross section, open at both its feed end and its        discharge end, the external surface of the said tubular metal        body having, in the vicinity of the nodal zone, a collar which        is coaxial with the said tube and which projects radially, this        collar being equipped at its periphery with an ultrasonic        converter which projects radially and whose frequency is equal        to the vibration frequency of the said collar and to the        longitudinal vibration frequency of the said tubular metal body,        and    -   a filtering cylindrical element having a minimum filtration area        of about 50 cm² and preferably about 80 cm² with a mesh opening        less than about 20 μm, which is arranged inside the tubular        metal body and coaxially with the latter between its feed end        and discharge end, the nominal ultrasonic power density        dissipated inside the tubular metal body being greater than        about 2 watts per cm² of filtering area.

According to another feature of the invention, the length of the tubularmetal body is equal to a whole number of half wavelengths of theultrasonic vibration frequency delivered by the converter, the internaland external diameters of the collar being determined so that thevibration frequency of the said collar is at the same ultrasonicvibration frequency as that delivered by the converter.

According to another feature of the present invention, the filteringcylindrical element is chosen from woven or nonwoven metal filtersmounted on a metal support, woven or nonwoven synthetic filters mountedon a metal support, sintered multilayer metal filters andself-supporting sintered metal or mineral filters.

According to a particular feature of the invention, the filteringcylindrical element is connected to the tubular metal body by connectingparts designed such that the liquid to be filtered passes, either fromthe outside of the filtering element towards the inside thereof, or,conversely, from the inside of the filtering element towards the outsidethereof.

According to a particular embodiment of the invention, the filtrationdevice may be integrated into a circuit carrying a liquid to befiltered, making it possible to measure the pressure difference betweenthe inlet and the outlet of the device and to control this pressuredifference by adjusting the flow rate of liquid in the circuit and/orthe pressure drop downstream of the said device.

According to another variant of the invention, the connecting partsbetween the tubular metal body and the filtering cylindrical elementtogether with the circuit carrying the fluid to be treated are designedsuch that the filtration is carried out according to a tangentialfiltration principle.

In general, the filtration device according to the invention may beintegrated into a circuit carrying a liquid to be filtered, making itpossible to carry out periodic counter-current cleaning of the filteringelement, as soon as the pressure difference between the inlet and theoutlet of the said filtering element exceeds a predetermined thresholdvalue.

The invention also relates to a method of using a filtration device asdescribed above, according to which the intensity of cavitation in theliquid to be filtered can be adjusted by controlling the power of theultrasonic vibration and/or by the choice of the shape of a boosterinterposed between the ultrasonic transducer and the external surface ofthe said collar which projects radially in the vicinity of the nodalzone of the said tubular metal body.

Finally, the present invention allows the construction of an ultrasonicfilter made by coupling, in series or in parallel, a plurality ofmodular ultrasonic treatment units as described above containing acoaxial filtering cylindrical element, thus creating an in-linemulti-filter device. Such an ultrasonic filter may comprise severalconverters powered in parallel by the same generator.

The present invention will be described hereinbelow in more detail withreference to two particular embodiments illustrated by the appendedfigures in which FIGS. 1 and 2 correspond to a first embodiment of thesimple filtration device and FIG. 3 corresponds to an embodiment fortangential filtration. In these figures and in the rest of thedescription, corresponding elements are denoted by the same references.

As shown in FIGS. 1 to 3, the modular reactor unit 12 mainly consists ofthree essential characteristic elements. First of all, the unit 12comprises a tubular metal body 11 having an internal cylindrical surface13 of circular cross section. The tubular metal body 11 is open at itstwo ends, that is to say at its feed end 10 and at its discharge end 15.These two feed and discharge ends 10, 15 will be coupled to feed anddischarge pipes, themselves possibly fitted with circulation pumps. Thispart of the plant is not shown given that it calls on entirelyconventional components well known to a person skilled in the art.

The modular unit 12 also comprises, on the outer surface of the tubularmetal body 11, in the vicinity of the nodal zone of the latter, a collar17 which is coaxial with the said tube, the said collar projectingradially outwards from the free surface of the tubular body 11.

Finally, the modular unit 12 comprises at least one ultrasonic converter19 which is arranged radially and in such a way as to be integral withthe said collar 17 at the periphery thereof. The frequency of the saidconverter 19 is equal to the vibration frequency of the said collar 17and to the longitudinal vibration frequency of the tubular metal body11.

In practice, a conventional converter, for example of the type withpiezoelectric excitation, is used for example for the ultrasonicconverter 19. It may for example be of the “Langevin Triplet” type, asis described in the work High Intensity Ultrasonics by B. Brown and J.E. Goodman.

According to the particular embodiment shown in FIGS. 1 to 3, thecoaxial collar 17 is machined directly from the tubular metal body 11.In this type of embodiment, the collar 17 is connected to the externalsurface of the tubular body 11 via connection fillets 21. The tubularmetal body 11 has, according to the embodiment described, a length equalto a half wavelength for the frequency that it is desired to use. Itshould be noted in this respect that the frequency of the ultrasonicvibration delivered by the emitter or converter 19 will also be between5 and 100 kHz. In this particular embodiment, the length of the tubularmetal body is exactly equal to a half wavelength of the frequency of theultrasonic vibration. However, it is perfectly possible, within thescope of the present invention, to use a tubular metal part of largersize, which is extended, for example, on one or both sides of the axialcollar 17 by a length equal to a whole number of half wavelengths of thedelivered frequency, it being possible for the bond between this metalpart and the modular unit to be produced, for example, at thelongitudinal amplitude antinodes (stress nodes) by means of screwthreads, force fittings, welds or the like or even be machined from thesolid.

In a particularly advantageous embodiment, the modular ultrasonictreatment unit 12 could be a SONITUBE 20 or 35 kHz marketed by Sodeva.

In the simple filtration assembly illustrated in FIGS. 1 and 2, a pump(not shown) pumps the liquid to be filtered into one of the ends of themodular ultrasonic reactor 12. The liquid to be filtered thereforeenters the filtering element 14 via the part 16 or 18. If the inlet partis the part 16, the liquid is filtered by the filtering element 14 fromthe inside towards the outside, before emerging through the holes 20formed in the part 18.

The pressure gradient inside the device may be controlled by adjustingthe flow rate of the pump (not shown), located upstream of the deviceand/or by adjusting the opening of a valve located downstream of thedevice. Pressure measuring gauges, located upstream and downstream ofthe device, may advantageously complete the device, and the whole may beinstalled, as the case may be, in an industrial plant operated by aprogrammable controller.

In another variant of the simple filtration assembly, the inlet partleading into the modular ultrasonic reactor is the part 18; in thisvariant, the liquid to be filtered enters the said reactor 12 throughthe holes 20 formed in the part 18, then is filtered by the filteringelement 14 from the outside towards the inside, before emerging throughthe part 16.

With reference to FIG. 3, in an example of a tangential filteringassembly, the blocking and connection means 22 have an additional holeallowing double communication of the tubular ultrasonic reactor 12 withthe external environment. A first communication takes place via thecentral hole 24 and puts the inside of the filtering element 14 incommunication with the external environment. A second communicationtakes place by the additional hole 26 and puts the outside of thefiltering element 14 in communication.

In this assembly, the liquid to be filtered enters the reactor 12through the holes formed in the part 28. The arrows shown in FIG. 3indicate the flow routes of the liquid. The liquid divides into twoparts: the first part passes through and is filtered by the filteringelement 14 from the outside towards the inside, before emerging from thereactor 12 through the central hole 24 of the part 22, while the secondpart of the liquid flows tangentially to the filtering element 14 beforeemerging from the said reactor through the additional hole 26.

Two valves located downstream of the device, one on the circuit of thecentral hole 24, the other on that of the additional hole, allow therespective flow rates of each circuit to be controlled.

Advantageously, when the present invention is placed within a liquidcircuit, it may be subject to periodic counter-current cleaning of thefiltering element, preferably triggered automatically, as soon as thepressure difference between the upstream and downstream ends of the saidfiltering element exceeds a predetermined threshold.

As a consequence of the above, the present invention continuouslyapplies ultrasonic vibration in order to improve the filtration process.Thus it allows a uniform distribution of the ultrasonic power in contactwith the filtering surface to be obtained.

A first exemplary use of the assembly, according to that illustrated inFIGS. 1 and 2, has been used satisfactorily in practice.

The tubular ultrasonic reactor is a standard SONITUBE 35 kHz by Sodevawith a length of 225 mm and an internal diameter of 20 mm, placed in avertical position.

The filtering element 14 used is of Poremet 5 μm absolute type; it hasan external diameter of 12 mm and a length of 225 mm.

A 15% by weight suspension of titanium carbide in water, in the form ofpowder less than 2 μm, is filtered by the invention in order to trap themanufacturing residues and the agglomerates. The suspension is pumpedfrom a stirred reservoir, with a flow rate of 10 liters per minute. Itflows through the tubular ultrasonic reactor 12 from the bottom upwardsand passes through the filtering element 14 from the inside towards theoutside.

Particle size analysis of the suspension shows that the fines content isgreater than 99% by mass of the solid phase. However, withoutultrasound, the upstream pressure increases rapidly and the liquid stopspassing through the filtering element 14 after about 1 minute. The saidfiltering element is clogged.

With ultrasound, the pressure remains constant at every point of thecircuit, the pressure drop within the tubular ultrasound reactor beingless than 100 mb. At the end of 12 minutes, that is 120 liters ofsuspension or 21 kg of carbide powder, the filter has retained 3.6 g ofparticles but has kept its porosity.

In a second example, the tubular ultrasonic reactor is a standardSONITUBE 20 kHz by Sodeva with a length of 370 mm and an internaldiameter of 50 mm, placed in the vertical position and assembled fortangential filtration, valves located downstream of the said filteringelement allowing the pressure difference between the upstream anddownstream ends of the said filtering element to be varied.

The filtering element used is a Poremet 10 μm filter with a diameter of40 mm and a length of 30 mm.

A 10% by mass suspension in water of ground silica in the form of apowder having particle sizes between less than 1 μm and 40 μm, isdivided up by the invention, by cut-off at 10 μm.

Firstly, the suspension is deagglomerated in the lower part of thetubular ultrasonic reactor not having a filter. A portion of the liquidpasses through the filtering element with most of the particlesmeasuring less than 10 microns while a smaller portion of the liquidcarries the other particles and remains outside the filtering elementbefore leaving the said reactor. During the operation, it is found thatthe pressures remain constant until the 50 liters of suspension areexhausted.

A third example uses a tubular ultrasonic reactor 12 of standardSONITUBE 35 kHz type by Sodeva with a length of 225 mm and an internaldiameter of 20 mm, placed in the horizontal position, according to oneembodiment as illustrated in FIGS. 1 and 2.

The filtering element 14 is a Poremet 2 μm filter, with a diameter of 12mm and a length of 225 mm.

The process involves filtering a 5% by mass suspension in water ofalumina powder intended for polishing. The objective is to remove theparticles greater than 2 μm.

The suspension is pumped at a flow rate of 5 liters per minute. Thepressure difference between the upstream and downstream ends of thefiltering element does not exceed 1 bar after 20 min, in spite of theformation of a layer of residue of about 3 mm around the filteringelement.

A simple counter-current operation without ultrasound enables theresidue to be recovered while regenerating the filtering element byremoving all the particles trapped in the pores of the said filteringelement.

1. Continuous filtration device in a liquid path, comprising: a modularultrasonic treatment unit (12) which is in the form of a tubular metalbody (11) with a cylindrical internal surface (13) and of circular crosssection, open at both its feed end (10) and its discharge end (15), theexternal surface of said tubular metal body having, in the vicinity of anodal zone, a collar (17) which is coaxial with said tube and whichprojects radially, this collar being equipped at its periphery with anultrasonic converter (19) which projects radially and whose frequency isequal to the vibration frequency of said collar (17) and to thelongitudinal vibration frequency of said tubular metal body (11), and afiltering cylindrical element (14) having a minimum filtration area ofabout 50 cm² with a mesh opening less than about 20 μm, which isarranged inside the tubular metal body (11) and coaxially with thelatter between its feed end (10) and discharge end (15), the nominalultrasonic power density dissipated inside the tubular metal body beinggreater than about 2 watts per cm² of filtering area.
 2. Deviceaccording to claim 1, wherein the length of the tubular metal body (11)is equal to a whole number of half wavelengths of the ultrasonicvibration frequency delivered by the converter, the internal andexternal diameters of the collar (17) being determined so that thevibration frequency of said collar is at the same ultrasonic vibrationfrequency as that delivered by the converter (19).
 3. Device accordingto claim 1, wherein the filtering cylindrical element (14) is chosenfrom woven or nonwoven metal filters mounted on a metal support, wovenor nonwoven synthetic filters mounted on a metal support, sinteredmultilayer metal filters and self-supporting sintered metal or mineralfilters.
 4. Device according to claim 1, wherein the filteringcylindrical element (14) is connected to the tubular metal body byconnecting parts designed such that the liquid to be filtered passesfrom the outside of the filtering element towards the inside thereof. 5.Device according to claim 1, wherein the filtering cylindrical element(14) is connected to the tubular metal body by connecting parts designedsuch that the liquid to the filtered passes from the inside of thefiltering element towards the outside thereof.
 6. Device according toclaim 1, wherein the device is integrated into a circuit carrying aliquid to be filtered, making it possible to measure the pressuredifference between the inlet and the outlet of the device and to controlthis pressure difference by adjusting the flow rate of liquid in thecircuit and/or the pressure drop downstream of the said device. 7.Device according to claim 1, wherein the connecting parts between thetubular metal body and the filtering cylindrical element together withthe circuit carrying the fluid to be treated are designed such that thefiltration is carried out according to a tangential filtrationprinciple.
 8. Device according to claim 1, wherein the device isintegrated into a circuit carrying a liquid to be filtered, making itpossible to carry out periodic counter-current cleaning of the filteringelement, as soon as the pressure difference between the inlet and theoutlet of the said filtering element exceeds a predetermined thresholdvalue.
 9. Device according to claim 1 wherein the minimum filtrationarea is about 80 cm².
 10. Ultrasonic filter, made by coupling, in seriesor in parallel, a plurality of modular ultrasonic treatment unitsaccording to claim 1, containing a filtering cylindrical element so asto create an in-line multi-filter device, said filtering cylindricalelement integrated into a circuit carrying a liquid to be filtered,making it possible to carry out periodic counter-current cleaning of thefiltering element, as soon as the pressure difference between the inletand the outlet of the said filtering element exceeds a predeterminedthreshold value.
 11. Ultrasonic filter according to claim 10, whereinthe filter comprises several converters powered in parallel by the samegenerator.
 12. Method of using a filtration device according to claim 1,comprising adjusting an intensity of cavitation in the liquid to befiltered by controlling the power of the ultrasonic vibration and/or bythe choice of the shape of a booster interposed between the ultrasonictransducer and the external surface of said collar.